Current tissue equivalent detectors are primarily proportional counters. These devices are ion chamber like devices that are filled with a tissue equivalent gas, where the charge collection is “amplified” by a strong electric field that is produced in a high voltage region of the detector. The dimensions of its active region are orders of magnitude larger than those of typical cellular dimensions. In order for these devices to function as microdosimeters it is necessary to have an appropriate product of the density of the active region and the characteristic dimension (diameter for spherical detectors). Gas-filled proportional counter type detectors therefore are filled with a low-pressure combination of gases that have the correct proportion of tissue equivalent elements (primarily carbon, hydrogen, and oxygen).
There are a number of known problems associated with these proportional counters. Most notable are:
Typically, they require the presence of a vacuum system aintain the appropriate gas mixture. While sealed devices can be constructed, they are generally subject to leakage over time.
Since the devices are large, and the charge multiplication and collection is happening in a low-pressure gas, they require high voltage for efficient operation.
Since the overall size of these devices is orders of magnitude larger than living cells, they will collect events over a significant macroscopic volume. Their response to individual particles would be microdosimetrically correct. However the possibility exists that two or more particles may be incident on the active region at “the same time” even if those incident particles are separated by distances on the order of millimeters (or perhaps more). The response of the gas filled detector to this class of events will be microdosimetrically incorrect.
To overcome these drawbacks, it was disclosed in my prior patent, U.S. Pat. No. 6,278,117, to employ a solid state photoconducting detector that senses ionizing radiation and which is primarily made out of organic material that has a density very close to that of normal tissue. Microstructures were used to make a semiconductor out of a polymer and to measure the current through the polymer without relying on long range conduction. The invention of U.S. Pat. No. 6,278,117 arranged the microstructures in a particular geometry that reduced the capacitance and the noise associated with large capacitance. In particular it placed the polymer on the top of a double layer which reduces the capacitance of the system. The detector's efficiency for any given radiation quality and energy could be measured. Once this was done, the detector was able to directly measure dose equivalence.
In particular, the invention disclosed in U.S. Pat. No. 6,278,117, provided a tissue equivalent solid state detector comprising a polymeric substrate having on its surface, by deposition or other means, a metallic hinder layer. A metallic electrode layer contacted the metallic binder layer. An active polymeric layer was cast onto the polymeric substrate, so that the metallic electrode layer was embedded in the active polymeric layer. The metallic electrode layer had at least two interdigitated conductor lines, each leading to a wire such that there was a small capacitance between the pair of wires. In operation a source of potential was place across the wires and the resistance and/or current across the conductors was measured with an electrometer, bridge or electronic monitoring circuit.
The invention disclosed in U.S. Pat. No. 6,278,117 further comprised a method for manufacturing such a detector. The method uses photolithographic techniques on a polymer surface by the steps of printing interdigitated metal patterns on the substrate by a liftoff process, sputtering a hinder layer and a metal electrode layer onto the substrate, peeling or lifting off the metal layers to leave the interdigitated patterns of polymer substrate, dicing the substrate so that each interdigitated pattern set becomes one die detector, bonding a die detector into a tissue equivalent case, bonding wires to permit connecting the die's bonding pads to external connections, and applying a polymer as the active region of the device. The polymer used was selected from among polythiophene, polyanaline, polyphenylene, and polyphenylene vinylene polymers.